Enzymatic browning is one of the most studied reactions in fruits, vegetables and seafood. Researchers in the fields of food science, horticulture, plant physiology, including postharvest physiology, microbiology and insect and crustacean physiology, have studied this reaction because of the diversity of its commercial impact upon growers, food processors and consumers.
Many of the research programs have demonstrated successful formulations for preserving processed fruit, vegetables, fish, poultry and meat, but these formulations have proven commercially non-viable due to, inter alia, the expense and/or limited availability of intermediates.
Appearance, flavour, texture and nutritional value are four attributes considered by consumers when making food choices. Appearance, which is significantly impacted by colour, is one of the first attributes used by consumers in evaluating food quality.
When asked to discuss discolouration or browning in foods, those involved from production to processing, usually reflect on its detrimental influence.
Discolouration or browning in fruits and vegetables also gives rise to economic losses. Increases in fruit and vegetable markets projected for the future will not occur if enzymatic discolouration or browning is not understood more and controlled. Enzymatic discolouration and browning is one of the most devastating reactions for many exotic fruits and vegetables, in particular tropical and subtropical varieties. It is estimated that over 50% of losses in fruit occur as a result of enzymatic discolouration or browning (Whitaker and Lee, 1995). Such losses have prompted considerable interest in understanding and controlling phenol oxidase enzymes in foods. Lettuce, other green leafy vegetables, potatoes and other starchy staples, such as sweet potato, breadfruit, yarn, mushrooms, apples, avocados, bananas, grapes, peaches, and a variety of other tropical and subtropical fruits and vegetables, are susceptible to discolouration or browning and therefore cause economic losses for the agriculturist. These losses are greater if discolouration or browning occurs closer to the consumer in the processing scheme, due to storage and handling costs prior to this point.
The control of discolouration or browning from harvest to consumer is therefore very critical for minimising losses and maintaining economic value to the agriculturist and food processor. Discolouration or browning can also adversely affect flavour and nutritional value of fruit and vegetables.
Decolouration, e.g. browning, of fresh produce, such as, fruit and vegetables is undesirable, especially for retailers and customers. Decolouration is anaesthetic and perceived by consumers to indicate that the produce is spoiled. Therefore, processors and retailers aim to prevent or minimise decolouration.
Such decolouration will generally not occur in undamaged or unprepared produce. However, there is an increased demand for prepared fruits and vegetables and therefore the prevention or mitigation of decolouration of such prepared foods is especially important for the retailer of such produce. If the produce is discoloured then the consumer will generally not purchase the product as it is perceived as being damaged.
Enzymic browning is an important colour reaction in fruit and vegetables and in some instances enzymic browning is desirable, for example in developing the flavour of tea and developing the colour and flavour in dried fruits such as figs or raisins.
However, enzymatic browning of many fruits and vegetables may be undesirable and can create economic losses for growers, retailers, etc. This decolouration or browning does not occur in undamaged or uncut fruit and/or vegetables since natural phenolic substrates are separated from the enzyme(s) responsible for browning hence the decolouration will not occur. However once the produce has been cut, peeled, damaged so that the flesh of the fruit or vegetable is exposed to air, rapid decolouration or browning will occur. This discolouration or browning, of produce such as fruit and vegetables, is often referred to as “enzymic browning” or “enzymatic browning”. Enzymic/enzymatic browning comprises a chemical or biochemical process which involves the enzyme polyphenol oxidase (phenolase), and other enzymes, such as, tyrosinase and catecholase. The enzyme is released when the fruit or vegetable is cut or damaged and discolouration is generally due to enzymic oxidation of phenols to orthoquinones, etc. the orthoquinones very quickly polymerise to form coloured/brown pigments known as melanins. Melanins are a class of pigments which are derived from the amino acid tyrosine and it is the melanin, or similar compounds in its class, which produces the brown colour observed in fresh produce as hereinbefore described.
The increase in the sale of pre-prepared fruits and vegetables has increased the need for the prevention of discolouration so as to increase at least the perceived shelf life of such produce.
Conventionally, enzymatic browning is controlled with chemicals (such as sodium bisulphite), or by destroying the responsible chemicals with heat, for example, blanching is commonly used destroy the enzyme(s) and to preserve the colour in fruit and/or vegetables. Lemon juice and other acids have been used to preserve the colour in fruit, particularly apples, by lowering the pH.
However, it is important to understand the discolouration process more fully. To enable the discolouration to occur four essential components are required to be present:                1. Oxygen        2. Enzyme        3. Prosthetic group—Copper        4. Substrate        
Therefore to introduce some control of enzymic browning one or more of these components needs to be eliminated from the reaction. A number of considerations need to be observed in order to understand what preventative action can be initiated to minimise browning.
Oxygen
Removing oxygen is both difficult and impractical. Fresh produce, such as fruit and vegetables, requires oxygen to maintain normal, or a degree, of respiration. Furthermore, removal of oxygen can favour the growth of anaerobic pathogenic organisms, such as Clostridium perfringens, Clostridium botulinum and Listeria monocytogenes. 
Enzyme
Phenolase enzymes are not easily removed from fresh produce. Heating or blanching treatment has been used for many years, whilst this does inhibit phenolase and other enzymes, it also causes undesirable softening and may itself cause the formation of black discolouration, for example, as in potatoes often referred to as “after cooking darkening” or “ACD”
Copper
Copper or iron accelerates the rate of the enzymatic browning. This can be easily observed when fruit is cut with a rusty knife or mixed in a copper bowl. The use of a chelating agent, such as, EDTA, phosphate based compounds, e.g. sodium acid pyrophosphate, and citric acid have been investigated, but have generally been unsuccessful in preventing phenolase decolouration.
Polyphenol Oxidase (PPO)
Polyphenol Oxidase (PPO) enzymes catalyse the o-hydroxylation of mono-phenols (phenol molecules in which the benzene ring contains a single hydroxyl substituent) to 0-phenol molecules containing two hydroxyl substituent's). They can also further catalyse the oxidation of o-diphenols to produce o-quinones. It is this rapid polymerisation of o-quinones to produce black, brown or red pigments (polyphenol) that is the cause of discolouration or browning in fruit and certain vegetables.
The amino acid tyrosine contains a single phenolic ring that may be oxidised by the action of PPO to form o-quinones. Hence PPO may also be referred to as tyrosinase. Polyphenol Oxidase is found in fruit, e.g. apples, and is the enzyme responsible for the fruit turning brown. However discolouration or browning is not unique to apples. A mixture of mono-phenol oxidase and catechol oxidase enzymes is present in nearly all plant tissue and can also be found in bacteria, animals and fungi.
Action of Polyphenol Oxidase

However, a disadvantage with many conventionally known anti-browning agents is their inability to penetrate fruits and/or vegetables quickly.
Sulphites, such as sodium metabisulphite, are known to penetrate fruits and vegetables, quickly and have been used extensively with root vegetables, such as potatoes. However, the use of sulphites is disadvantageous in that, when opening sulphite treated pre-packed vegetables, such as potatoes, there can be a “whiff” of a sulphurous odour.
Other potential anti-browning agents have been investigated including, for example, anti oxidants, acidulants, chelating agents, enzyme inhibitors and inorganic salts. However, many of them suffer from the disadvantage that they are expensive and/or not commercially available.
U.S. Pat. No. 5,939,117 describes a calcium ascorbate composition which is suitable as an anti-browning composition however, such a composition is disadvantageous in that, inter alia, large amounts of ascorbate are required to be use which is undesirable and costly.